The use of the reverberatory furnace for the smelting of nonferrous metal sulfides to a nonferrous matte, the matte subsequently being converted and refined to recover the valuable nonferrous metal therefrom, has been one of the primary means for the recovery of metals such as copper, nickel and the like. Numerous such plants for the production of those nonferrous metals are in existence and operation, although recent developments with respect to energy and the environment have created problems relative to use of reverberatory furnaces.
Such reverberatory furnaces are horizontal vessels having a refractory lining, and burners at one end, with the internal width of such a vessel being about twenty-five to thirty-five feet, the internal length being generally about one hundred feet, and a heighth between the hearth and roof thereof of between about ten and fifteen feet. Roof construction varies but is commonly of suspended basic or sprung silica design. The furnace is fossil fuel-fired through burners at one end, although various placements of such burners throughout the furnace may be used, the burners combusting oil, natural gas or pulverized coal to heat a charge of material within the furnace and effect smelting of the sulfides to matte. Exhaust gases are normally discharged at the end of the furnace opposite the burner end. The furnace design generally provides for slag tap-holes, at or near the end of the furnace opposite the burners; while the matte tap-holes are variously located. Charging of sulfide concentrate and flux to the furnace is usually accomplished by side feeding along the furnace walls.
Reverberatory furnaces, as known in the art and as employed worldwide, are wasteful consumers of fossil fuels and, in addition, harm the environment. Such furnaces, as typified in the smelting of raw copper sulfide concentrates, suffer from serious inefficiency in heat transfer and as a chemical reactor. The same holds true even if the furnace feed is hot roaster calcine rather than wet filter cake. These furnaces must be supplied with large quantities of natural gas, oil or coal, which have now greatly increased in cost, and may be in short supply or better used for higher priority requirements.
The dusty off-gases from conventional reverberatory furnaces are high in volume and low in sulfur dioxide content, e.g., one percent. The former results in high cost of dust recovery while the sulfur dioxide content is too low for economical sulfur fixation, yet too high for environmental acceptance as discharge to the atmosphere. The cost of dust recovery is directly related to the gas volume requiring treatment. Also, a feed stock of at least about four percent sulfur dioxide is required for efficient operation of a sulfuric acid plant, and much preferably eight percent, for reasons of economy. Alternative sulfur fixation means require even richer sulfur dioxide feed streams for economic viability.
The process fuel efficiency of conventional reverberatory furnace operation is low, primarily because gas-solid contact is poor and hence the rate of heat exchange between the hot gases and the charge fed down the side walls of the furnace is low. As a result, as much as half of the fuel's heat content escapes in the furnace exhaust gas. Chemical reaction efficiency is low because not only gassolid contact but gas-liquid and liquid-liquid contact are also poor. The heat and mass transfer characteristics of the reverberatory furnace are poor because the active surface to mass ratio of the furnace input components is small. Thus, furnace performance is sluggish. It wastes energy in all its forms, in addition to its adverse impact on the environment.
In view of the high cost of replacing reverberatory furnaces with apparatus and processes of more advanced design, many studies and experiments have been carried out both by industry and by government in search of improvements in reverberatory furnace practice to decrease air pollution, especially SO.sub.2 emission, to conserve fossil fuel, and to improve metallurgical efficiency. One alternative which has been extensively investigated, but with generally unfavorable outcomes, is flue gas scrubbing, e.g., using lime slurries to remove SO.sub.2 in the form of a calcium sulfate sludge. Another costly approach to treatment of reverberatory furnace gases is to first concentrate its SO.sub.2 content by absorption in an organic solvent, followed by fixation of the concentrated SO.sub.2 as elemental sulfur, sulfuric acid, or liquid SO.sub.2. Because the bulk of the volume of reverberatory flue gas is fossil fuel combustion products, including the nitrogen from the air supplied for combustion, substitution of commercial oxygen for part of the air supplied to the reverberatory furnace burners for fossil fuel combustion has been adopted in some installations increasing fuel efficiency and permitting higher smelting throughput ratio. Lancing of oxygen through the roof of reverberatory furnaces, to increase smelting capacity and SO.sub.2 content of off-gas while reducing fuel consumption, has also been examined but has not achieved commercial success. Problems which can be encountered include damage to the refractories by excessive localized heating and splashing of the bath. Lances can be employed to obtain good gas-liquid-solid contact in rotary converters by creating a turbulent bath. This was disclosed by one of the present inventors in U.S. Pat. Nos. 3,004,846; 3,030,201; 3,069,254; 3,468,629; 3,516,818; 3,605,361 and 3,615,362. However, use of lances to adapt the turbulent bath principle to reverberatory furnaces is not practical. Summarizing the above discussed and other previously proposed modifications of existing reverberatory furnaces and auxiliary processes, none have gained wide acceptance and none seem capable of postponing the abandonment of most of these furnaces.
Advanced technology for the treatment of nonferrous sulfide concentrates involves complete abandonment of the reverberatory furnace for smelting purposes along with some or all of the ancillary equipment. Examples are the new Noranda and Mitsubishi continuous smelting processes. A recent development by the present inventors is the Q-S Oxygen Process for continuous, autogenous conversion of nonferrous metal sulfides to matte or metal as described in U.S. Pat. No. 3,941,587, wherein autogenous conversion is effected in a single reactor with introduction of oxygen effected above and beneath the molten bath.
Two flash smelting processes, i.e., the INCO oxygen flash smelting process and the Outukumpu Oy process, are well established alternatives to the conventional reverberatory furnace process and employ furnaces of special design. In INCO oxygen flash smelting, as disclosed by one of the applicants herein, in U.S. Pat. NO. 2,668,107, the sulfide-flux-oxygen mixture is injected into a reverberatory type furnace, of special volume enclosed in an impermeable steel casing, through horizontally disposed end burners. These burners which are similar to conventional pulverized coal burners, inject the dry solid charge with oxygen as a jet-like stream.
In conventional practice, reverberatory furnaces are the primary smelting apparatus for nonferrous mineral concentrates. The substitution of an advanced technological process may be difficult for economic reasons. Nevertheless, the continued use of such reverberatory furnaces, as hereinbefore described, had taken on grave disadvantages in respect to both energy and environmental conservation.
An object of the present invention is to provide a process for application in existing reverberatory furnaces and which overcomes several of the drawbacks currently associated with their use.
Another object is to provide a process that enables the use of existing reverberatory furnaces, with relatively simple and inexpensive alterations and additions, to smelt nonferrous mineral sulfides to matte, at greatly increased throughput rates, accompanied by greatly decreased fuel rates and greatly increased sulfur dioxide content of the furnace exhaust gas.
A further object of the present invention is to provide a method whereby tonnage oxygen can be skillfully employed so as to allow ready replacement of standard, obsolete reverberatory furnace practice by a relatively efficient and economic smelting procedure. In fact, Examples IV and V hereinafter indicate that the process of the present invention is competitive with the two flash smelting processes now in commercial use. This invention permits postponement of the heavy capital expenditures otherwise required for total plant replacement so as to comply with government energy and environmental conservation regulations.
It is an additional object of the present invention to provide a process for smelting of nonferrous mineral sulfides whereby pulverized coal may be judiciously employed in minor quantities to effectively control the value metal content of both matte and slag output.